According to the uncertainty principle of quantum physics, a quantum state exists in an indeterminate state. Similar to the wave-particle duality of light, it only collapses into a definite state in reality when an observer is present.
The famous quantum physicist Schrodinger once proposed a thought experiment involving a cat. A cat is placed in a box containing a small amount of radium and deadly cyanide. The decay of the radium element is governed by probability.
If the radium decays, it triggers a mechanism that shatters the vial of cyanide, killing the cat. If the radium does not decay, the cat lives on.
According to the theory of quantum mechanics, since the radioactive radium exists in a superposition of both decayed and non-decayed states, the cat should logically also exist in a superposition of being dead and alive. In other words, the cat is simultaneously dead and alive.
